In view of conservation of global environment, development of fuel cells which have excellent power generation efficiency without emitting CO2 has been promoted in recent years. This type of fuel cells generates electricity by an electrochemical reaction between H2 and O2. The basic structure of the fuel cell is an electrolyte membrane (in other words an ion exchange membrane), two electrodes (in other words a fuel electrode and an air electrode), diffusion layers for O2 (or air) and H2, respectively, and two separators. Depending on the kind of electrolyte membrane, there have been developed phosphoric-acid fuel cells, molten carbonate fuel cells, solid-oxide fuel cells, alkaline fuel cells, proton-exchange membrane fuel cells, and the like.
As of these fuel cells, the proton-exchange membrane fuel cells have advantages including the ones given below compared with other types of fuel cells:
(a) The power generating temperature is about 80° C., allowing generating electricity at a significantly low temperature level;
(b) Design of reduced weight and size of the fuel cell body is available; and
(c) Start up within a short time is achieved.
Owing to these advantages, the proton-exchange membrane fuel cells currently draw keenest attention among various types of fuel cells as a power source of electric vehicles, a stationary power generator for household or commercial services, and a portable miniature power generator.
The proton-exchange membrane fuel cell generates electricity from H2 and O2 via a polymer membrane. As illustrated in FIG. 1, the proton-exchange membrane fuel cell has a structure of sandwiching a membrane-electrode assembly 1 between gas diffusing layers 2 and 3 (carbon paper and the like) and further between separators 4 and 5, thus forming a single structure unit (what is called a single cell), thereby generating electromotive force between the separator 4 and the separator 5.
The membrane-electrode assembly 1 is called “MEA”, which has an integrated structure of a polymer membrane being sandwiched between the respective electrode materials such as carbon black sheets supporting a platinum-based catalyst, giving total thicknesses from several tens of micrometers to several hundreds of micrometers. The gas diffusing layers 2 and 3 are often integrated with the MEA 1.
When the proton-exchange membrane fuel cell is used for above-applications, those single cells are combined in series by the quantities from several tens to several hundreds to form a fuel cell stack.
The separators 4 and 5 are required to have functions of:
(A) Separation wall between single cells;
(B) Electric conductor to transfer the generated electrons;
(C) Air flow channel and hydrogen flow channel through which O2 (in other words air) and H2 flow, respectively; and
(D) Discharge flow channels for discharging the generated water and gas, respectively.
Furthermore, practical applications of the proton-exchange membrane fuel cells require to use the separators 4 and 5 which have high durability and electric conductivity.
For the case that the proton-exchange membrane fuel cells are used as the power source of electric vehicles, the expecting durability is about 5000 hours of service life. For the case of utilizing them as the household stationary power generator or the like, the expecting durability is about 40000 hours of service life. Accordingly, the separators 4 and 5 are requested to have corrosion resistance enduring the long period of power generation service because once the corrosion occurred to elute metallic ions, the proton conductivity of the electrolyte membrane decreases.
Regarding the electric conductivity, the separators 4 and 5 and the gas diffusion layers 2 and 3 are wanted to have low contact resistance as far as possible because the increase in the contact resistance of the separators 4 and 5 and the gas diffusion layers 2 and 3 decreases the power generation efficiency of the proton-exchange membrane fuel cell. That is, smaller contact resistance of the separator and the diffusion layer gives superior power generation characteristics.
Currently a proton-exchange-membrane fuel cell utilizing graphite as the separators 4 and 5 has been brought into practical application. The separators 4 and 5 made of graphite have advantages of relatively low contact resistance and of non-corrosiveness. Since, however, they are readily broken under impaction, they have drawbacks of difficulty in scale down and expensive working cost for forming the air flow channel 6 and the hydrogen flow channel 7. Those drawbacks of the separators 4 and 5 made of graphite hinder the wide utilization of the proton-exchange membrane fuel cells.
There are trials to adopt metallic base material as the base material of separators 4 and 5, instead of the graphite. Particularly in view of improvement of durability, various studies are given to the practical use of separators 4 and 5 made by stainless steel, titanium, titanium alloy, and the like as the base material.
For example, JP-A-8-180883, (the term “JP-A” referred to herein signifies the “Unexamined Japanese Patent Publication”), discloses a technology of using a metal such as stainless steel and titanium alloy, which easily forms a passive film thereon, as the separator. The formation of passive film, however, induces increase in the contact resistance, which results in decrease in the power generation efficiency. Therefore, those metallic base materials have various issues such as larger contact resistance than that of graphite base material and inferiority in corrosion resistance.
JP-A-10-228914 discloses a technology to decrease the contact resistance and to assure high output power by applying gold plating on the surface of the metallic separator made of austenitic stainless steel (SUS304 in other words type 304) or the like. The thin gold plating, however, is difficult to prevent pinhole generation, while thick gold plating arises a cost problem.
JP-A-2000-277133 discloses a method to form a separator having improved electric conductivity (or decreased contact resistance) by dispersing carbon powder particles in the matrix of ferritic stainless steel. Since, however, the surface treatment of the separator needs a cost even with the use of carbon powder, there still remains the cost problem. In addition, the separator after surface treatment has a problem of significant decrease in the corrosion resistance once flaws or other defects appear during assembly stage.
There are trials of using stainless steel as the separator without applying surface treatment. For example, JP-A-2000-239806 and JP-A-2000-294255 disclose ferritic stainless steels for the separator, adding positively Cu and Ni, and decreasing impurities such as S, P, and N, while satisfying [(C+N)≦0.03% by mass], and [10.5% by mass≦(Cr+3×Mo)≦43% by mass]. JP-A-2000-265248 and JP-A-2000-294256 disclose ferritic stainless steels for the separator, limiting Cu and Ni to 0.2% by mass or less to suppress elution of metallic ions, decreasing impurities such as S, P, and N, while satisfying [(C+N)≦0.03% by mass], and [10.5% by mass≦(Cr+3×Mo)≦43% by mass].
Those inventions, however, are based on a concept that the ingredients of stainless steel are limited to a specific range to strengthen the passive film, thereby suppressing the deterioration of performance of the catalyst supported on the electrode caused by the eluted metallic ions even without applying surface treatment, and suppressing the increase in the resistance of contact with electrode caused by the corrosion products. Accordingly, these inventions do not intend to decrease the contact resistance of the stainless steel itself. Furthermore, they cannot secure the durability to prevent the reduction in the output voltage during service period of several tens of thousands of hours.
There are also studies of the effect of surface roughness of a separator on the contact resistance. For example, JP-A-2002-270196 discloses a proton-exchange membrane fuel cell which uses a separator made of a stainless steel having surface irregularities, and which stainless steel is covered by a Cr-enriched passive film on the surface thereof. According to the disclosure, a preferred range of the surface roughness parameter is from 0.03 to 2 μm of centerline average surface roughness Ra, in other words arithmetic average surface roughness. According to our finding, however, even stainless steels having equivalent Ra with each other give significantly different contact resistance from each other so that sole maintaining Ra in a specific range is difficult to significantly decrease the contact resistance.
Responding to the above problems of the related art, it could therefore be helpful to provide: a metallic material for conductive member, which gives good corrosion resistance and small contact resistance (in other words high electric conductivity), specifically a metallic material for conductive member, such as stainless steel, titanium, and titanium alloy to easily form a passive film thereon, in particular a metallic material for a separator in a proton-exchange membrane fuel cell; a separator using thereof; and a proton-exchange membrane fuel cell using the separator.
That is, it could be helpful to provide: a metallic material for a separator in a proton-exchange membrane fuel cell, which gives small contact resistance, high power generation efficiency, and high corrosion resistance of the metallic material itself, without applying surface treatment such as gold plating, by specifying not only the components of the metallic material having a property to easily form a passive film thereon but also, among various surface roughness parameters, the surface roughness parameter significantly affecting the contact resistance with the gas diffusion layer to a specific range; a separator using the metallic material; and a proton-exchange membrane fuel cell using the separator.